Reduces the apparent grain in an image digitised
from film. Film grain tends to have certain features exploited by
RemGrain, which will not necessarily work with images that don't come
from film!
INPUT CLIP
The image from which grain is to be removed.
Tips:
- The grain reduction process works best if at least the first image of a clip has a
largish rectangular region that contains a roughly flat color. This is often the case for
green or blue screen shots.
- Grain reduction is done in three stages:
- The colors in the flat background region are looked at statistically, to get an estimate
of how noisy a supposedly constant-ish bit of the image is. These statistics are used in a
later stage.
- One of the color records of the film is often much noisier than the other two, i.e. the
blue. The first grain reduction step is to conditionally smooth the noisiest of the three
records.
- The resulting image is then further smoothed in all color records where a weighted sum
of the color records has less than a threshold variance (specified relative to the
background region variance). In the sum, much greater weight is given to the quiet
records, so the result is that smoothing is only done in areas where the original image is
likely to have few sharply defined features. This reduces the grain in smooth areas of the
image. This procedure makes use of the characteristics of real film, and the fact that
grain is hard to see in truly detailed parts of the image. It is only apparent on smooth
regions.
- The execution time is independent of any of the user controls. It is important that the
background region is sensibly set. If it is poorly chosen, or threshold is very low, the
entire image may be blurred.
SELECTION AREA
Use the XY Min/Max controls to select an area of the image, ROI, which represents a
flatish area of colour in the image. If you select part of the action then
the grain found will be confused with detail. You need only sample the first
frame in order to analyse the the grain statistics. See Tips above.
Minimum XY and Maximum XY (Default: left hand corner)
Defines the region in the image which will be analysed for grain.
Retain (Default: Off)
Off=0: the statistics will be found for each frame.
On=1: once the first frame has been processed, the statistics found in
the ROI, will be used in processing the rest of the clip.
Threshold (Min: 1.0 Max: 5.0 Default: 2.0)
If the standard deviation of pixel colors in a small region of the input image around the
current output pixel is less than Threshold times the standard deviation in the
background region, grain reduction processing will be done for that output pixel.
Typically, the higher this figure, the more grain reduction will be performed. If it is
set too high, important image features may be blurred. If too low,
most of the grain may be left intact.
Red Dominance (Min: 0 Max: 20 Default: 3)
Green Dominance (Min: 0 Max: 20 Default: 2)
Blue Dominance (Min: 0 Max: 20 Default: 1)
Controls how much we can trust a color record of the film to represent real features in
the image. For most films, red is quieter than green, and blue is very much the noisiest.
The color record weights should give least importance to blue, and most to red for such a
film.
Index